Numerous radio wave direction finding systems are known in the prior art. An early example of a direction finding system is the spaced loop direction finding system described in 1921 by Eckersly. In 1935, Eckersly and Marconi described an improved direction finding system based on a precision spaced loop which rotated. More recent sophisticated direction finding systems are shown in U.S. Pat. No. 3,093,827.
The previous fixed spaced loop systems were designed to provide accurate azimuth angle of arrival for incoming ground wave signals. However, their performance was degraded for sky wave signals having significant elevation angles with horizontally polarized electric field components. The direction finding system shown in U.S. Pat. No. 4,207,572 was designed to provide accurate azimuthal and elevation measurements on the incident signals, both for ground waves and also for multipolarized sky waves independent of elevation.
An electrically small loop is equivalent to a magnetic dipole, and can be used in a receive mode to provide partial information about electromagnetic polarization and consequently about direction of arrival for ground wave signals. Both spinning loops and fixed simple loop arrays have been successfully employed for ground wave direction finding. A properly arranged array of simple loops can also be connected to produce an equivalent magnetic quadrupole. Such magnetic quadrupoles have also been successfully used for groundwave direction finding, but suffer polarization/elevation limitations very similar to the simple loop systems.
Performing direction finding on HF radio signals which have propagated through the ionosphere (sky wave signals) requires DF systems that are free of the above described intrinsic polarization errors. Typically, such systems have been large aperture systems. However, a suitable site that is sufficiently large to accommodate large aperture systems is frequently unavailable at the desired location. To provide skywave direction finding capability with small aperture systems, several spaced loop DF systems have been developed for operation against skywave signals. These include spinning spaced loops and arrays of multiple fixed spaced loops that equivalently synthesize spinning spaced loops. The spinning spaced loop systems do not lend themselves readily to automated, unattended operation, and have the shortcomings that are typical of mechanically moving system. The skywave spaced loop array is a system of six independent outputs and requires rather complex processing to produce the unambiguous bearing.